3 Special PCB Wiring Layout
PCB board has three kinds of special wiring layout techniques, namely the right-angle alignment, differential alignment and serpentine alignment.
Influences caused by right-angle alignment are mainly reflected in the three aspects: first, the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time; second, the impedance discontinuity will cause the reflection of the signal; third, the right-angle tip can generate EMI. All these small right-angle may be the trouble maker for high-speed issues.
What is a differential signal? Generally speaking, the driver sends two equal-valued and inverted signals. The receiver determines whether the logic state is "0" or "1" by comparing the differences between the two voltages. The pair that carries the differential signal is called a differential alignment. Compared to common single-ended signal alignment, the most obvious advantages of differential signals alignment are strong anti-interference ability, effective EMI suppression, accurate timing positioning.
Serpentine is a type of alignment that is often used in PCB layout. Its main purpose is to adjust the delay to meet the system timing design requirements. Two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). It is obvious that when the signal is transmitted on the serpentine alignment, the parallel segments will be coupled and form a differential mode. The smaller the S, the larger the Lp, the greater the degree of coupling. This may result in reduced transmission delays and greatly reduced signal quality due to crosstalk.